The present invention relates to a technology which is effectively applied to a transmitter circuit in a communication semiconductor integrated circuit device constituting a wireless communication system and further a transmitter circuit of such a modulation method that phase modulation and amplitude modulation are carried out. More particularly, the present invention relates to a technology effectively utilized in a transmitter circuit of direct up conversion type wherein a modulator which carries out modulation into an oscillation signal (carrier wave) on a predetermined frequency by I-signal and Q-signal carries out direct conversion into transmission frequency.
Conventionally, various cellular phones have been proposed. One example is a dual band cellular phone capable of handling signals in two frequency bands, for example, GSM (Global System for Mobile Communication) signals in the 900-MHz band and DCS (Digital Cellular System) signals in the 1800-MHz band. Recently, demand has grown for a triple band cellular phone capable of handling, for example, PCS (Personal Communication System) signals in the 1900-MHz band in addition to the GSM and DCS signals. For high frequency ICs used in a cellular phone supporting a plurality of bands, sharing of VCO is effective in terms of reduction of a number of parts.
The conventional GSM, DCS, and PCS use a phase modulation method designated as GMSK (Gaussian Minimum Shift Keying) wherein only the phase of carrier waves is shifted according to transmission data. As a one-chip high frequency IC which supports only GSMK modulation and is capable of transmitting triple band signals, one based on a method designated as offset PLL has been already brought into practical use.
The offset PLL transmitter circuit modulates an I-signal and a Q-signal by an oscillation signal on an intermediate frequency (fIF). Thereafter, the offset PLL transmitter circuit supplies an oscillation signal on a higher frequency (fRF) than that of the carrier wave and an oscillation signal (fTX) for transmission outputted from a transmission oscillation circuit (TXVCO). The transmitter circuit supplies these oscillation signals to a mixer designated as offset mixer, and causes the mixer to output a signal corresponding to the difference (fRF−fTX) in frequency between the two signals. Then, the oscillating operation of the TXVCO is feedback-controlled so that the frequency of this differential signal is matched with the frequency of the modulating signal. In this method, both transmission signals in the 900-MHz band and transmission signals in the 1800-MHz band can be coped with by one RFVCO by varying the frequency dividing ratio of the RF signal supplied from the mixer. Therefore, the method is excellent in terms of reduction of a number of parts.
For recent cellular phones, the EDGE (Enhanced Data Rates for GSM Evolution) method has been proposed. In the EDGE method, communication of audio signals is carried out by GMSK modulation, and data communication is carried out by 3π/8 rotating 8-PSK (Phase Shift Keying) modulation. The 8-PSK modulation is modulation wherein amplitude shift is added to phase shift of carrier waves in the GMSK modulation. While information of one bit is sent per symbol in the GMSK modulation, information of three bits can be sent per symbol in the 8-PSK modulation. For this reason, the EDGE method can carry out communication at a higher transmission rate than the GSM method.
However, in the offset PLL transmitter circuit, when the control terminal of TXVCO is supplied with a feedback signal, the oscillation frequency is changed but the oscillation amplitude is constant. Therefore, though the offset PLL method is suitable for a transmitter circuit supporting GSM which carries out only phase modulation (GMSK modulation) it cannot be applied to a transmitter circuit supporting EDGE involving amplitude modulation.
One of methods for implementing a modulation method wherein both the phase component and the amplitude component of a transmission signal are made to bear information is polar loop. The polar loop method is such that: a signal to be transmitted is separated into a signal containing a phase component and a signal containing an amplitude component. Thereafter, they are controlled by feedback by phase control loop and amplitude control loop, respectively, and then synthesized in an amplifier and outputted. However, the polar loop transmitter circuit has a problem. Since the transmitter circuit is provided with two control loops, phase loop and amplitude loop, its control system is complicated. This makes it difficult to reduce the size of a high frequency IC chip and its cost.
Other methods for implementation include two-step up conversion and direct up conversion. In the two-step up conversion, an I-signal and a Q-signal are modulated by a signal on an intermediate frequency (IF), and then converted into transmission frequency. In the direct up conversion, an I-signal and a Q-signal are directly modulated by a signal on a transmission frequency. The inventions related to such methods are disclosed in, for example, Patent Document 1 and Patent Document 2.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-013246
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2002-094331